Browser defaults and polyphils

https://www.codecademy.com/courses/learn-intermediate-css/lessons/learn-intermediate-css-browser-compatibility/exercises/browser-defaults

trying to make sense of browser defaults

i found these in the exercise

https://necolas.github.io/normalize.css/8.0.1/normalize.css

i have no clue what i am looking at here

help me out?

please address

same request for Polyfills

https://www.codecademy.com/courses/learn-intermediate-css/lessons/learn-intermediate-css-browser-compatibility/exercises/polyfills

i know this is a lot

many thanks!